The present invention relates to a fast atom beam source and, more particularly, to fast atom beam source structure that is capable of emitting a fast atom beam efficiently at a relatively low discharge voltage.
As is well known, atoms and molecules undergo a thermal motion in the atmosphere at room temperature when possessing a kinetic energy of about 0.05 eV. "Fast atoms" are atoms and molecules that have a kinetic energy over about 0.05 eV, and when such particles are emitted in one direction, they are called "fast atom beam".
FIG. 2 shows exemplarily a fast atom beam source that emits argon atoms with a kinetic energy of 0.5 to 10 keV, among conventional fast atom beam sources designed to generate fast beams of gas atoms. In the figure, reference numeral 1 denotes a cylindrical cathode that also serves as a casing, 2 a doughnut-shaped anode, 3 a DC high-voltage power supply, 4 a gas nozzle, 5 argon gas, 6 plasma, 7 fast atom beam emitting-holes, and 8 a fast atom beam.
The constituent elements, exclusive of the high-voltage power supply 3, are incorporated in a vacuum container (not shown).
The conventional fast atom beam source comprising the above-described constituent elements operates as follows.
After the vacuum container has been sufficiently evacuated, argon gas 5 is injected into cylindrical cathode 1 from the gas nozzle 4. Meanwhile, a high DC voltage is impressed between the anode 2 and the cathode 1 by the DC high-voltage power supply 3 in such a manner that the anode 2 has a positive potential, and the cathode 1 a negative potential.
As a result of the above-described process, electric discharge occurs between the cathode 1 and the anode 2 to generate plasma 6, thus producing argon ions and electrons. During this process, electrons that are emitted from the inner wall surface at one end of the cylindrical cathode 1 are accelerated toward the anode 2 and pass through the central hole in the anode 2 to reach the inner wall surface at the other end of the cathode 1. The electrons reaching the inner wall surface at the second end lose their speed. Then, the electrons are turned around and are accelerated toward the anode 2 to pass again through the hole of the anode 2 before reaching the inner wall surface at the first end of the cathode 1. Such repeated motion of electrons forms a high-frequency vibration between the two end faces of the cylindrical cathode 1 across the anode 2, and while making the repeated motion, the electrons collide with the argon gas to produce a large number of argon ions. The argon ions produced in this way are accelerated toward each end face of the cylindrical cathode 1 to obtain a sufficiently large kinetic energy. The kinetic energy obtained at this time is, for example, about 1 keV when the discharge sustaining voltage impressed by the DC high-voltage power supply 3 is 1 kV. In other words, there is a turning point of electrons vibrating at high frequency in the vicinity of each end face of the cylindrical cathode 1. This point is a region where a large number of electrons with low energy are present. Argon ions change to argon atoms in this region by collision and recombination with the electrons. In the collision between the ions and the electrons, since the electrons are so much smaller than that of the argon ions that their mass can be considered negligible, the argon ions deliver the kinetic energy to the atoms whose charge changes without substantial loss, thus forming fast atoms. Accordingly, the kinetic energy of the fast atoms is about 1 keV. The fast atoms accelerated are emitted in the form of a fast atom beam 8 to the outside through the emitting holes 7 provided in the second end face of the cylindrical cathode 1.
The above-described conventional fast atom beam source suffers, however, from some problems described below. To increase the rate of emission of the fast atom beam, the prior art needs to raise the discharge voltage, use a magnet jointly with the described arrangement, or increase the pressure of the argon gas introduced and cannot adopt any other method, as can be understood from the above-described matters, that does not result in an increase in the energy of the fast atom beam, or an increase in the overall size of the apparatus, or an extension in the energy band of the fast atom beam, etc. Thus, the prior art presents many problems and difficulties in use.